Reassociate loop invariants involved in Cmps and Add/Subs

Following the ideas of https://bugs.openjdk.org/browse/JDK-8250808, we can reassociate expressions involving Cmps and Add/Subs

```
// Apply the same transformations to == and !=
inv1 == (x + inv2) => ( inv1 - inv2 ) == x
inv1 == (x - inv2) => ( inv1 + inv2 ) == x
inv1 == (inv2 - x) => (-inv1 + inv2 ) == x
```

This can only be done for `==` and `!=`, or else we can break comparisons if there are overflows.